LONDON — African migrants in Libya face “unimaginable horrors,” the United Nations human rights commissioner declared. “Despicable,” the chairman of the African Union called their treatment. Several African countries recalled ambassadors in protest. Rwanda offered the migrants assistance.

The mid-November broadcast by CNN showing what was described as African migrants being auctioned off at a Libyan slave market — for as little as $100 each, at black-market exchange rates — has set off an international firestorm. The response from the European Union, however, has been notably muted.

That may partly reflect the gratification among European Union officials over Italy’s success at reducing the influx of migrants across the Mediterranean. Italy has been helping Libyans stop them at sea or keep them in Libya, despite the dangers they face there.

Rights groups and other experts say the video of the slave market — although no surprise to many journalists or relief workers — is an uncomfortable reminder for Europe that its policies risk trapping the migrants in slave-like conditions.

“The tragic and morally unjustifiable thing about this is that European Union policy is certainly a part of why this is happening,” John Springford, who studies migration at the Center for European Reform, a research organization in London. “But whether that will lead to a change in direction, I am doubtful.”

Marco Minniti, the interior minister of Italy and the architect of its new Libya migrant policies, was asked about the slave market video in a parliamentary hearing, and argued that Italy had also done what it could to provide humanitarian aid.

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Migrants whose dinghy was sinking tried to board a Libyan coast guard ship during a rescue operation off the coast of Libya in November.CreditLisa Hoffmann/Sea-Watch, via Associated Press

“Is that enough?” he said. “Of course not. But the alternative cannot be to just accept the impossibility to govern the migration flux and hand to the human smugglers the keys to the European democracies.”

The migrant crisis in Libya originated with the collapse of the government of Col. Muammar el-Qaddafi six years ago. The near-total absence of policing since then has attracted hundreds of thousands of migrants from other African countries hoping to cross by boat to Italy, or at least be rescued by a European ship that would deposit them there.

But the chaos and lawlessness across Libya has also exposed the migrants to pervasive and well-documented abuses, including forced labor, kidnapping, extortion, rape, torture and indefinite extralegal detention in overcrowded pens and other inhumane conditions.

Traffickers who take money to transport the migrants from their countries of origin often hold them hostage and demand more money once they reach Libya. Then traffickers pack migrants into flimsy and overcrowded vessels that often sink.

Many passengers die of drowning or dehydration, and survivors picked up by Libyan vessels or in Libyan waters are often packed into prisonlike detention centers.

Both the traffickers and the militiamen running the detention centers may sell migrants into forced labor or sexual exploitation, and the traffickers often bribe detention centers to regain the captured migrants so that they can extort more money from friends and family in their country of origin.

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Migrants in an overcrowded boat before they were rescued on the Mediterranean Sea in January.CreditSima Diab/Associated Press

“It is a kind of hell,” said Vickie Hawkins, executive director of Doctors Without Borders, the medical charity, which has been visiting migrant detention centers in the area around Tripoli for more than a year.

Even with the travel risks, more than half a million migrants made it to Italy over a three-year period ending last December, and that has led to a populist backlash there and elsewhere in Europe. Responding to the pressure, Italy, with the support of the European Union, took new steps this year to halt the exodus.

The Italian government struck deals with Libyan militias to hold back migrant departures. It provided patrol boats to the loosely controlled and often-untrained Libyan forces operating as a coast guard. And the Italian Navy dispatched a ship to conduct surveillance and locate migrant boats before they depart Libyan waters.

The Italian government has also sought to apply public pressure to discourage aid groups from depositing rescued migrants in Sicily, while the increasingly assertive and sometimes dangerous Libyan coast guard has deterred humanitarian rescue ships from plying migrant boat routes.

The monthly rate of migrant arrivals in Italy began falling sharply in July. By October, the number had fallen by more than two-thirds, to fewer than 6,000, down from more than 27,000 in the same month a year earlier, according to the International Organization for Migration, an intergovernmental agency. The total number of arrivals in the first eleven months of the year fell by more than 30 percent, to about 117,000 from about 173,000 in the same period last year.

“We have a real chance of closing the central Mediterranean route,” Donald Tusk, president of the European Union’s governing council, said in late October at a summit meeting in Brussels.

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Migrants arriving at a naval base in Tripoli after they were picked up by the Libyan coast guard in November.CreditHani Amara/Reuters

But the migrants are trapped in increasingly overcrowded conditions. Ms. Hawkins of Doctors Without Borders said her group had seen a tenfold increase in the number of detainees in the eight centers where it has worked, to 12,000 in recent weeks from as few as 1200 in July.

“I visited a detention center in July that had 300 men in it and it already seemed crowded,” she said. “Three weeks later there were 800, and I could not fathom how you could fit 500 more in there.”

The European policy “is looking an awful lot like complicity,” said Judith Sunderland, a researcher for Human Rights Watch. The sale and exploitation of African migrants in Libya has been well known “for a very long time, frankly,” she said.

As early as the spring of 2015, journalists who visited migrant detention camps in Western Libya reported that the jailers routinely sold captives to local farmers or others for temporary use as laborers.

“We are like slaves,” Abu Bakr Dixon, 34, a Gambian held at a detention center in Zawiyah, said at the time.

SOS Mediterranean, an organization that rescues migrants attempting to cross the sea, has published repeated testimony over the last 21 months from Africans who said they had been bought or sold by Libyans — typically by the militiamen running the detention centers.

Last April, the International Organization for Migration published a report documenting the existence of Libyan “slave markets” where dark-skinned migrants were auctioned, and it was widely covered at the time by international news organizations.

The report said that traffickers who had taken payment to transport African migrants across the desert for passage to Europe instead sold their human cargo to Libyans, and that the Libyan buyers often resold them to others.

“We get so used to hearing the words, ‘I was sold,’” said Meron Estafanos, a human rights advocate based in Stockholm who works with migrants from her native country, Eritrea. “It makes you feel, where were all these people when we were reporting it before?”

After the expressions of shock from around the world at CNN’s slave-market video, many Libyans accused the Europeans of hypocrisy for having acted surprised.

“The international community keeps turning a blind eye to facts when it comes to #Libya & #migration,” Emad Badi, a Libyan activist in Tripoli, wrote on Twitter. “Newsflash: there is no ‘coast guard’, slavery is not new, interception at sea breeds internal problems, expecting the nationally contested #GNA to handle migration is foolish.” He was referring to the Western-backed Government of National Accord, one of three rival governments vying for power in Libya.

Taher el-Sonni, an adviser to the president of the Western-backed government, wrote on Twitter: “Trafficking = Slavery, to put the blame only on Libya is unacceptable!”

Federica Mogherini, the top diplomat for the European Union, has been measured about the abuses, but has acknowledged that European policymakers have long known about the enslavement of migrants in Libyan detention centers. “We have to face the dramatic situation of our African brothers and sisters that are in slavery in those centers,” she said recently at a meeting with African diplomats in Brussels.

Without defending the migrant-return policies, Ms. Mogherini emphasized that this year the European Union has also started providing support for flights returning some migrants waylaid in Libya to their home countries — as many as 15,000 by the end of the year, she projected.

“This is only a drop in the ocean, but it is the first time that we have done this,” she said.

A version of this article appears in print on , on Page A9 of the New York edition with the headline: Europe Wanted Migrants Stopped. Now Some Are Sold as Slaves.. Order Reprints | Today’s Paper | Subscribe